1. Field of the Invention
This invention relates to thick film resistive element heaters and more specifically to a thick film heater with a metal substrate where the metal has a high coefficient of thermal expansion such as aluminum.
2. Related Art
As used herein, "Thick Film" means a metal based paste containing an organic binder and solvent, such as ESL 590 ink, manufactured by Electro-Science Laboratories, Inc., Philadelphia, Pa. ("ESL"). "Ceramic Oxide" means a refractory type ceramic having a high content of oxidized metal; "MPa" means mega Pascals (large units of Pressure); "Coefficient of thermal expansion (10E.sup.-6 /.degree. C.)" (CTE) means micro-units of length over units of length per .degree. C. or parts per million per .degree. C.; and "W/m.multidot.K" means watts per meter kelvin (units of thermal conductivity). High expansion metal substrates means ferrous or non-ferrous metal having a CTE of 16.times.10E.sup.-6 /.degree. C. or higher.
Thick film resistive element heaters are relatively thick layers of a resistive metal based film as compared to "thin film" technology (1-2 orders of magnitude thinner than thick film) and is typically applied to a glass based dielectric insulator layer on a metal substrate when used as a heater.
Heaters having a body or substrate made of a metal with a CTE of greater than 16.times.10E.sup.-6 /.degree. C. such as high purity aluminum or high expansion stainless steel are desirable. This is because aluminum or other like metals have excellent thermal conductivity properties which makes it an ideal substrate or body for heaters requiring extraordinarily uniform temperature distribution. However, for metals that have excellent thermal conductivity and uniform heat distribution characteristics, as noted, it is also not unusual for these metals to have higher CTEs like aluminum. Conventionally, aluminum heaters are made by embedding a coil heating element inside an aluminum cast or by putting a foil heater beneath an aluminum plate with an insulation material such as a mica plate in between. Aluminum heaters of this type can have a thinner profile than comparably rated heaters made of steel. The thinner profile is achievable while maintaining the desired heater performance because of the high thermal conductivity of aluminum which is 10-20 times higher than standard 400 series stainless steel. However, as in the case of aluminum, there is also a high CTE.
The profile of the heater can be reduced even further if the heater comprises a metal substrate with a "thick film" heating element applied to the substrate because thick film technology allows precise deposition of the heating element at an exact location where heat is needed and intimate contact of the heating element to the substrate which eliminates any air gap there between. Another benefit of using thick film is that there is a greater flexibility of circuit designs to better achieve uniformity in temperature distribution and to provide precision delivery of heat for better control and energy savings. Also, thick film resistive elements can be made to conform to various contoured surfaces required for specific custom applications.
Thick film heaters are typically applied on top of a glass dielectric material that has already been applied on the metal substrate. It is desirable to utilize a glass dielectric in combination with thick film technology because glass based materials provide a very flat and smooth electrically insulated surface layer, glass materials are not porous, and are not moisture absorbing. These characteristics of glass materials allow the thick film to be applied easily while achieving the desired trace pattern and with the correct height or elevation and width of the trace.
Thick film heating elements are desired because thick film can offer uniform temperature distribution because of the flexibility to form various small or intricate heating element trace pattern designs. Therefore, a thick film on an aluminum substrate would be very useful if it could be made to work because of aluminum's thermal performance characteristics. So far the prior art teaches the use of a glass based dielectric when using thick film over a metal substrate, but that will not work when using aluminum as the substrate metal or other metals having a high CTE relative to the typical glass dielectric utilized with thick film. Therefore, even though the thermal performance of aluminum is desirable, the high CTE is not compatible with a glass based dielectric. As seen in industry, thick film heaters on metal substrates use glass dielectric material to serve as an insulation between the thick film and the metal substrate, usually 400 series stainless steel which has a CTE of 12.times.10E.sup.-6 /.degree. C. The reason why aluminum or other higher CTE metals are problematic is aluminum has a much higher thermal expansion coefficient than glass used for 400 series stainless steel and therefore causes cracking in the glass dielectric material when heating or cooling occurs. The cracking causes opens in the resistive heating film resulting in a defective heater. Cracking typically occurs when the aluminum substrate is cooling down and contracting after the temperature has been raised. A second problem is that the typical printing method for applying such a dielectric is screen printing which requires a firing post-process for the curing of the dielectric. The melting point of aluminum is about 600.degree. C. Therefore, if a glass dielectric is utilized, it must have a lower melting point than 600.degree. C. in order to be properly fired for adequate curing. A glass having a low melting point of 600.degree. C. can be found, but the final heater design will be limited to a low operating temperature (below 400.degree. C.). This is because the softening temperature of a glass dielectric is usually 200.degree. C. or more lower than the melting temperature (hypothetically 600.degree. C. --in order to work with aluminum). Also, when glass reaches its transition temperature, which is 50-100.degree. C. below the softening temperature, the glass will significantly loose its insulation resistance properties. Therefore, just above the softening temperature, the glass will significantly loose its insulation resistance properties, so the heater is limited to temperatures below 300.degree. C. This renders an aluminum-glass heater design useless for many applications. In addition, the dielectric cracking problem is not resolved by choosing a glass dielectric with a lower melting point. A third problem is that if a glass with a lower melting point is chosen then the firing temperature to cure the thick film element applied on top of the dielectric is limited to that of the glass. Therefore a special thick film must be found that has a lower curing or sintering temperature.
The above problems have prevented the use of thick film heater elements on aluminum substrates because, even if a thick film with a lower melting point (lower than the melting point of the glass dielectric chosen) is found and utilized, the resulting operating temperature of the heater would be useless for many operating temperatures and the dielectric cracking problem is still not resolved because the difference in the coefficient of thermal expansion still exists. Also, a glass based dielectric with such a low melting point will have poor insulation performance at the higher operating temperatures and insulation breakdown is likely.
Conventional wisdom then is that aluminum or other higher CTE metals like high expansion stainless steel is simply an incompatible substrate for thick film heaters.